Submarine fault scarps in the Sea of Marmara pull-apart ... - HAL - IRD

Geochemistry
Geophysics
Geosystems G 3
G 3 armijo et al.: sea of marmara pull-apart 10.1029/2004GC000896
not be older than a century. The higher average
sedimentation rates (up to 3 mm/yr) determined
over the whole Holocene period with the 14 C dates
of wood fragments in the long cores (see Figure 3)
also supports this inference.
[44] From the historical seismicity in the region
[Ambraseys and Finkel, 1995; Ambraseys and
Jackson, 2000], the large earthquakes of 1912 or
of August 1766 are the only ones that can be
considered consistent with the location, the rupture
length and the slip associated with the
described submarine break. However, only the
1912 event is consistent with the youthful morphology
and the fast sedimentation rates in Marmara.
Remains of ruptures of the 18th century
earthquakes would probably be less intact, less
continuous and largely concealed by sediments.
Regardless, there appears to be fragmentary evidence
for a penultimate event of similar size in
1766 on the SE Tekirdag fault, based on the
cumulative offsets recorded on the visible scarps
(Figures 8b and 9d). That hypothesis seems
quantitatively consistent with the pattern of intensity
values assigned for the 5 August 1766
earthquake [Parsons, 2004]. Our observations do
not provide support for the inference of seismic
quiescence over the last 500 years in the northwest
Sea of Marmara [Ambraseys and Jackson,
2000].
4.2. Previous Knowledge of the 1912
Earthquake
[45] The submarine rupture in the northwest Sea of
Marmara must be compared with the relevant
observations of the 1912 earthquake, which can
be summarized as follows.
[46] The 9 August 1912 Ganos earthquake produced
widespread damage and considerable loss of
life. Surface disruptions along the Ganos fault were
described in contemporary reports as ‘‘cracks’’
[Macovei, 1912; Mihailovich, 1927; Sadi, 1912].
No fault rupture was reported as such, but pictures
of mole tracks and en echelon cracks clearly
suggest prevalent right-lateral slip [Ambraseys
and Finkel, 1987]. The significant normal slip of
3 m deduced by Ambraseys and Finkel [1987]
appears to be mostly a secondary effect. The littoral
to the southwest of where the Ganos fault enters
the Sea of Marmara appears to have been slightly
uplifted [Ambraseys and Finkel, 1987]. Such uplift
is consistent with the transpressive deformation
seen in submarine scarps in the southwestern
Tekirdag Basin.
[47] Various authors have offered different isoseismal
maps. The most recent and complete revision
of macroseismic effects (estimates of local intensities
reportedly within an error of ±1/2 degree of
intensity) has been proposed by Ambraseys and
Finkel [1987] and is based mainly on the precise
observations by Mihailovich [1927]. Their two
isoseismal maps can be compared in Figure 11.
Ambraseys and Finkel [1987] deduced that coseismic
faulting crossed all the land between the Gulf
of Saros and Marmara with a total rupture length
of 50 km. The rupture could well have extended
westward some 20–30 km into the Gulf of Saros.
It is worth noting that the macroseismic effects
(within estimated error) suggest that the rupture
may have extended several tens of kilometers
under the western Sea of Marmara although
without constraining precisely its termination
(Figure 11).
[48] Small tsunami waves suggesting a possible
submarine extension of the rupture are reported
along the shores of the Sea of Marmara at the time
of the 1912 event but quantification of the phenomenon
appears difficult [Mihailovich, 1927;
Ambraseys and Finkel, 1987, 1991]. No tide gauge
record is available. Part of the difficulty comes also
from the scarcity of precise reports for an event that
occurred by night (main shock at 01:29 UTH;
about 03:30 local time).
[49] An accurate instrumental location is not possible
for the 1912 earthquake epicenter, due to the
primitiveness of seismic networks. However, a
reliable surface-wave magnitude Ms 7.4 ± 0.3
was calculated by Ambraseys and Finkel [1987]
from teleseismic amplitudes from 23 Milne pendulum
seismographs distributed worldwide and from
amplitudes and periods from 23 mechanical instruments
mainly from Europe. Hence the 1912 Ganos
and the Mw 7.4 1999 Izmit earthquakes are of
similar size. The 1912 earthquake was followed by
numerous aftershocks of which the largest occurred
a month later (Ms 6.9; 13 September 1912)
[Ambraseys and Finkel, 1987].
[50] On the basis of field investigations carried
out recently along the Ganos fault, Altunel et al.
[2000] and Altinok et al. [2003] claim that some
offset features associated with the 1912 event can
still be identified. Altinok et al. [2003] deduce a
maximum right-lateral slip of 4.5 m ± 0.5 m and
Altunel et al. [2004] describe a detailed coseismic
slip distribution along the Ganos fault and identify
three main segments. While the poor preservation
of the features may cause misidentifications
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